LIFE CYCLE OF HYDATINA SENTA 155 



this substance is absent that female-producers alone occur. Inas- 

 much as I used old food cultures (manure solutions) without caus- 

 ing any epidemic of male-producers, Whitney holds that the sub- 

 stance which does cause male-producers to appear is a transitory 

 product of decomposition of the manure, which is therefore pres- 

 ent only in new solutions, not in old ones He states his point 

 clearly (Whitney, '10, p. 348) : ''I would maintain that there seems 

 to be a definite but transitory chemical substance produced in 

 appreciable quantities in the decomposition processes in newly 

 made horse manure cultures that can so act upon the partheno- 

 genetic females as to cause them to produce sexual daughter 

 females. When this substance is absent, no sexual females are 

 ever produced, but only parthenogenetic females are produced 

 * * * ." In these words Whitnej^ clearly implies the belief 

 that no internal agent in Hydatina ever tends to produce sexual 

 females (male-producers) and that therefore whenever male-pro- 

 ducers appear they are a manifestation of some external agent. 

 I had assumed, on the contrary, that internal agents probably 

 tended to cause some male-producers to appear, without the direct 

 aid of any particular substances in the medium, and that the pres- 

 ence of certain substances prevented them. 



In further support of the view that some substance causing 

 male-producers to appear is present in new cultures but not in 

 old ones, Whitney cites his own earlier findings (Whitney, '07) 

 that the male-producers occurred predominantly in the earlier 

 parts of the family. Since the food culture was new in the first 

 part of the family and old in the latter part, one might expect that 

 some chemical difference between old and new food cultures 

 caused the male-producers to appear early in the family. That this 

 evidence can not stand seems probable from data presented 

 in the second of my papers (Shull, '10b), where it is shown, from 

 a large number of families, that male-producers do not occur more 

 abundantly in the first half of the family than in the last half. 



Whitney has not found any specific substances which cause 

 male-producers to appear. Search for them may well be success- 

 ful and should certainly be made. But it is now certain that the 

 absence of the substance which, he supposes, causes male-producers 



